A significant amount of effort has been undertaken to use existing telephone lines for high speed digital data communications. As part of this effort, a number of Digital Subscriber Line (DSL) systems have been proposed. For example, a version known as Asymmetric Digital Subscriber Line (ADSL) provides a system that applies signals over a single twisted-wire pair that supports "plain old telephone service" (POTS) and high-speed duplex (simultaneous two-way) digital services. Two of the proposed standards for ADSL are set forth in the ANSI T1.413 Issue 2 ADSL Standard (1998), and in the Universal ADSL Framework Document, Terms of Reference, Implemenation Guide (Jun. 15, 1998) from the Universal ADSL Working Group, both of which are hereby incorporated by reference. A third ADSL standard, proposed by the ITU, is the G.lite standard, further described in ITU Temporary Document NF-008, Draft G.lite Recommendation (May 11, 1998). Both of the proposed G.lite and U-ADSL (Universal ADSL) standards are variants of the T1.413 ADSL standard, with modifications directed primarily to work in a splitterless environment (i.e., without a splitter at the remote user end). A goal is to have both G.lite and U-ADSL merge into an identical (or at least very similar) standard in the future.
A DSL system essentially encodes digital data as analog signals at data rates significantly higher than voice band systems using special digital modems. Each user's link is conducted over twisted-pair conductors bundled with a large number of other twisted-pair conductors, each used at different times and for different purposes (e.g., voice only, data only, and both voice and data). The length and characteristics of wire run from a user's remote transceiver to a central office transceiver may vary greatly from user to user. In addition, the physical channel over which the system communicates varies over time due to, for example, temperature and humidity changes, fluctuating crosstalk interference sources, and, in splitteriess configurations, phones transitioning on-hook and off-hook. Consequently, the analog DSL signals exists in a noisy, time varying environment. Accordingly, DSL systems use sophisticated equalizer training, echo canceller training, and synchronization techniques (collectively, training) to cope with these factors, all of which require retraining from time to time. Additionally, DSL system equipment may go offline at any time, such as when powered down or placed into an idle or sleep mode. Retraining may be necessary or desirable to transition such equipment back online.
FIG. 1 is a block diagram of one embodiment of a prior art DSL system. A user's computer 10 is coupled to a DSL modem 12 through a band splitter 14 to a conventional telephone line 16 and thence to a telephone company (telco) system 18. The telco system 18 includes a DSL modem and necessary equipment to establish a link to, for example, the Internet. The splitter 14 separates voice band frequencies from higher data band frequencies. A conventional telephone 20 may be coupled to the splitter 14 for communication over the voice band frequencies.
FIG. 1 depicts a typical architecture for the T1.413 ADSL standard. The splitter 14 isolates the ADSL system from the effects of user telephone devices. Thus, a T1.413 compliant ADSL system does not require retraining to transition from an off-hook state to an on-hook state, or vise versa. However, for the proposed G.lite and U-ADSL standards, the splitter 14 at the remote end is optional, and the intention of the standards is that the splitter 14 will not be present. Under these latter standards, telephone on-hook/off-hook transitions have an adverse effect on the system and force retraining. Further, a T1.413 compliant ADSL system is "always on". However, G.lite and U-ADSL systems are "always available", and are actually put into low power states between sessions of user activity. "Waking up" from these low power states requires retraining.
At present, splitterless G.lite and U-ADSL DSL modems attempt to recover from a channel disruption or silent period (e.g., caused by an on-hook/off-hook transition or a dropped line) by using a fast retrain sequence. This procedure relies on the storage of profiles comprising stored modem operating parameters for particular line characteristics. During a fast retrain sequence between two transceiver units, if both units have maintained their stored profiles, then each unit's receiver may invoke a stored profile to set the modem characteristics for the far-end unit's transmitter, thereby permitting data transmission (named the "SHOWT1ME" state) to resume rapidly.
FIG. 2 is a diagram of the general timing sequence of an initialization training sequence for two transceiver units of a standard digital modem communication system in accordance with the prior art. One transceiver unit is a central office (C) unit; the other transceiver unit is a remote terminal (R) unit. The timing sequence includes the following steps:
(1) Activation: Either end may initiate a (re)training procedure. For T1.413 and U-ADSL, this involves the use of predefined activation tones which, when received by the far end, indicates a request to go through training and, if training is successful, enter the SHOWTIME state. For G.lite, the full train activation always uses G.hs signaling sessions (G.hs is a standard handshake procedure for initiating xDSL sessions by identifying common modes of operation and selecting and entering an appropriate mode), which can take 2 seconds or longer (but may be as short as about 0.5 seconds).
(2) Transceiver training: This typically includes setting receiver gain control, acquiring timing and training echo cancellers and equalizers.
(3) Channel analysis: The channel is measured to determine data handling capacity (in the form of a bits and gains table). For G.lite, the results of this (i.e., the bits and gains table) may be stored in a profile.
(4) Exchange: Negotiation and exchange of operating parameters as determined through channel analysis is performed (i.e., all the mandatory information typically stored in a profile is exchanged here--bits and gains table, Reed-Solomon coding parameters. data rates, etc.)
FIG. 3 is a diagram of the general timing sequence of a proposed fast retrain sequence for two transceiver units of a standard digital modem communication system in accordance with the prior art. The timing sequence includes the following steps:
(1) Activation: The G.lite fast retrain uses activation tones in a manner similar to T1.413 initialization.
(2) Channel measurement: The channel is measured to (a) ensure that the channel is stable (i.e., any transients due to on or off hook transitions have subsided), (b) determine power cut-back levels, and (c) begin characterizing the channel in order to select the appropriate profile (if one exists). Timing recovery should be performed here as well (the system may either fine-tune the timing if the previous state was the SHOWTIME state or may require timing if previous timing information is not available or no longer valid).
(3) Transceiver Retraining: Echo cancellers and equalizers are trained and timing, may be further tuned. Additional information stored in the profile from a previous session (such as previous equalizer taps) may be used to speed the process. In addition, some channel analysis may be done to assist with final profile selection.
(4) Profile Selection: Exchange of profile numbers. Profile numbers reference the information exchanged in a previous initialization Exchange procedure.
Regardless of whether or not the initiating or responding units can or wish to exchange profiles (e.g. because the profiles have been lost due to a power failure), nearly the entire fast retrain procedure must be completed before commencing the next desired operation (e.g., transition to a G.hs signaling session or performing an initialization training sequence where profiles are not required). Thus, if profiles do not exist, or there is otherwise a lack of information sufficient to return directly to a data transmission state at completion of the fast retraininig (i.e., the SHOWTIME state), or there are other reasons for truncating the fast retrain procedure, the process of completing a fast retrain sequence is futile or unnecessary. Such completion is inefficient and adds about 1.5 seconds or more to the start-up time of a DSL modem. For a system that is supposed to be "always on" or "always available", this time delay can seem significant to consumers.
The inventors have determined that it would be desirable in an ADSL system, and in particular in a splitterless ADSL system, if retraining could be accomplished rapidly to provide better performance. Accordingly, the present invention provides a method and system for improving performance in an ADSL system.